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Full text of "Handbook Of Chemical Engineering - I"

MECHANICAL SEPARATION                            289
most substances are charged negatively and hence move to the positive pole. Water itself is positive and moves toward the negative pole. No electro osmotic effects are noted in liquids like chloroform, ether, petroleum, oil of turpentine, etc. The addition of small quantities of electrolytes to solutions used hi cataphoresis may promote endosmosis or oppose it. Large additions cause the action to cease entirely.
The addition of OH ions to a negatively charged particle should raise the cata-phoretic migration velocity to the positive pole Acids and cations of higher valency would decrease it with increasing concentration and would finally change its sign (Freundlic, "Kapillarchemie," p. 239). Whitney and Blake (Jour. Am. Chem. Soc., 26, p. 1339) found that upon adding 0.004 normal nitric-acid solution to colloidal silver the particles originally migrating to the positive pole reversed their direction.
There appears to be no relation between the amount of material separated and the current used and the separations are hence not governed by Faraday's law (Tech. Paper 51, U. S. Bureau of Standards, Washington, 1915).
In some experimental work (op. cit.) at the Bureau of Standards, Washington, to test out the Schwerin invention and to determine whether clays submitted to direct current separation would be improved by it, the conclusion was reached that most clays were not improved in quality by the separation and the success of the invention would lie in its being able to compete with the filter press. There seems some ground for believing that the process can remove such substances as pyrite from clays which tend to form dark spots or patches in the burned articles and it may have use in the clay and ceramic industries for removing substances not mentioned in the test work.
In the experimental work at the Bureau of Standards the positive electrodes were copper cans and the other poles pieces of carbon slightly dipping into the water of the test tank. The positive electrodes used were respectively 5 and 6 in. in diameter and were set in small tanks. Finely ground flint separated well at 125-180 volts and 0.75 amp. No electrolyte was necessary. No separation with or without electrolytes on Brandywine feldspar. Maine feldspar did a little better. Finely divided ferric oxide formed no deposit with or without electrolytes. An addition of 6 per cent of the oxide was then made to North Carolina kaolin. No reduction of iron content resulted. This illustrates one of the peculiarities of this mode of separation and which lies in the fact that if there is one predominating substance of a mixture which will move to a pole it will carry the inert substance with it.
Runs were made with North Carolina and Florida kaolin. Heavy firm deposits were secured with 0.2 NaOH and voltages ranging from 80 to 160. All matter in suspension was carried to the electrode. The densest layer is obtained with the highest voltage.
A fire clay of the No. 3 type from Aultman, Ohio, was ground to pass the 40 mesh size and made into slip showing minimum viscosity with 0.1 per cent NaOH. The separation took place very satisfactorily.
Upon making briquettes from the original clay, the suspended portion and the residue the linear drying shrinkages were found to be 8.4, 7.5 and 6.25 per cent respectively. Upon burning the specimens to cone 9 in a sagger the separated portion was decidedly superior in appearance, showing a uniform gray color while both the original clay and the residual part showed numerous black spots.
The reverse effect of the negative electrode was also investigated. Upon making a steel trowel the negative pole, it was found that water was drawn from a mass of plastic clay into which the tool was inserted causing the latter to penetrate very readily. All the products tested were submitted to preliminary screening and settling operations to remove coarse particles. Sodium hydrate and sodium oxaiate were used as electrolytes.
A new process of water repelling-waterproofing of fabrics depends in part upon cataphoresis. The Tate electrical process of water-repelling waterproofing, as dis-